1. Field of the Invention
The present invention relates to a method, an apparatus, and a system for controlling data storage, and more particularly, to an apparatus for data storage control that duplicates data stored in a first storage device on a second storage device and a third storage device thereby preventing performance degradation that is caused by a shortage of a disk space of the first storage device used as a cache.
2. Description of the Related Art
A virtual tape system makes it possible to save data as if it were being stored on a physical tape device although it is actually stored on a disk device included in the virtual tape system (see, for example, Japanese Patent Application Laid-Open No. 2003-58326). In the virtual tape system, the disk device is used as a cache for high-speed data writing/reading.
The virtual tape system is generally connected to a tape library including a tape device at the back end of the system. Upon receiving data from a host device, the virtual tape system stores the data on the disk device as backup target data. Then, the virtual tape system backs up the data to a tape medium in the tape device at a predetermined timing. The backup target data is stored on the disk device until the data is completely backed up to the tape medium.
When the backup target data is highly significant, the data is usually backed up to a plurality of tape media. This method of backed up is referred to as “data duplication”. In the conventional technique, the backup target data is transmitted to all the tape media simultaneously in parallel.
Moreover, in case of emergency such as the occurrence of a natural disaster, the data is usually backed up not only onto the tape media but also to a tape medium present in a remote tape device and connected via a network. Namely, the backup target data is duplicated on the local tape devices as well as on the remote tape device.
Thus, it is necessary to duplicate data in case the data is lost due to a damage of the tape media or the occurrence of a natural disaster.
In the conventional technique, however, a time lag can occur among the tape devices at the time of data duplication. Namely, some of the tape devices may have different data writing times than others. The data writing times can differ, for example, due to a difference in level of performances (such as a data writing speed or a data transfer speed via an interface) or occurrence of hardware failures. Furthermore, in the remote tape devices, the data writing speed can particularly vary depending on a line speed of the network.
If a time lag occurs among the tape devices at the time of data duplication, some of the tape devices lag behind in backup of the data. The backup target data is held on the disk device until the backup target data is completely backed up to all the tape devices. Therefore, if massive amount of data is transmitted from the host device in clusters, the disk space of the disk device can run out. When the disk space of a disk device runs out, that disk device enters into a standby state; because, an area enough for data storage can not be secured in that disk device.
If disk devices frequently enter into a standby state, an overall speed of both the data writing and the data reading decreases. As a result, the virtual tape system fails to perform high-speed data writing or reading. Thus, there has been a need of a technology that can prevent performance degradation of a disk device caused by a shortage of the disk space.